This disclosure relates to a low density ultraviolet (UV)-curable optical fiber coating, fiber made therewith and a method of manufacturing the fiber.
It is sometimes desirable to use optical fibers having reduced density (i.e., reduced weight per unit length) for certain types of telecommunications applications such as, for example, those that require buoyancy for in-water or underwater sensing or guidance, e.g., in towed arrays for underwater acoustic sensing, marine antennas, or in a tethered vehicle (remotely operated underwater vehicle, ROV). They can also aid in the design and construction of buoyant fiber optic cables. Another application may be fibers for airborne fiber optic guided missiles. In general, there is a desire to reduce optical fiber weight in various applications, in order to reduce the energy used for its movement and transport. This includes, for example, fiber for air-blown fiber installation into ducts and fiber used in fluid-filled tubes in oil wells. It is further desirable that optical fiber coatings be UV-curable, so as to facilitate manufacturing of the fiber using well-known techniques.
For a glass optical fiber, the light-guiding component is generally a ceramic such as silica or doped silica, where the silica itself has a density of about 2.2 grams per cubic centimeter (g/cm3). This is significantly greater than water, having a density of about 1.0 g/cm3 (and of course, far heavier than air). To date, most known UV-curable optical fiber coatings have had densities greater than 1.0 g/cm3, i.e., themselves being denser than water. For commercially available UV-curable urethane acrylate optical fiber coatings, a typical primary (inner) coating has a density greater than or equal to 1.05 g/cm3 and a typical secondary (outer) coating has a density greater than or equal to 1.15 g/cm3. Some companies have reported or offered fibers having reduced-density polymer coatings where the reduction in density was achieved by either inducing a foam structure or by incorporating hollow microspheres into the fiber coating. However, a foaming process and the resultant foam density are difficult to control while still achieving geometric (wall thickness) control. Microspheres may settle (or float) in a liquid coating after incorporation and mixing, requiring tedious remixing to sustain uniformity. Microspheres, sometimes called microballoons, can be of a glass type or a polymer type. Glass microspheres, in particular, can be undesirably abrasive and potentially weaken a glass fiber surface by scratching if applied directly adjacent to a glass surface within a polymer coating. In addition, glass or polymer microspheres can also produce undesirable microbending when included in the fiber coating. And in cases where microspheres are incorporated within or on top of an outer coating layer, they tend to reduce the outer coating smoothness. This in turn increases frictional resistance when the fiber is dragged through either air or a fluid such as water, and such drag can be undesirable in certain applications.
Relatively few extrudable non-foamed thermoplastics are known to have densities less than 1.0 g/cm3 (e.g., polymethylpentene and certain types and copolymers of polyolefins such as polypropylene and polyethylene). However, it is difficult to extrude thermoplastics in thin layers in comparison to coating a fiber with a UV-curable resin because thermoplastics usually have higher resistance to flow; e.g., typical thermoplastic melt viscosities are high (greater than 100,000 centipoise) in comparison with the viscosity of a typical UV-curable optical fiber coating (about 4000 centipoise). Hence, use of low-density thermoplastics may limit the smallness of a coated fiber that can be readily achieved. Additionally, for the case of optical fiber coatings that are applied immediately adjacent to a glass surface, the coatings must be filtered to remove abrasive particles having size larger than about 1 micron in order to achieve sufficient fiber strength; such filtration is extremely difficult for resins having the high viscosities of most extrudable thermoplastics.